An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
However, a disadvantage associated with ink jet printers is that ink is ejected in discrete ink droplets; consequently, ink jet printers only print ink dots at discrete optical density levels. Therefore, unintended and extraneous ink dots give rise to undesirable image artifacts, which are most visible in low-density areas of the image on the receiver medium. Furthermore, due to the use of discrete optical densities, high bit-depth pixel values of a substantially continuous tone input digital image are necessarily "quantized" to the discrete density values available in a specific ink jet printer. However, when pixel values are quantized, "quantization errors" occur. These quantization errors produce image artifacts which may appear to the viewer of the image as posterization or contouring. Although quantization errors can be reduced by increasing the available levels of optical densities, such reduction of quantization errors increases printing time and ink laydown. Increased ink laydown on the receiver medium requires higher liquid absorption by the receiver medium, which is undesirable. Of course, increased printing time reduces the printer's production rate, which is also undesirable.
Methods for increasing available levels of optical densities are known. A method to increase available levels of optical densities is disclosed in U.S. Pat. No. 4,714,935 titled "Ink-Jet Head Driving Circuit" issued Dec. 22, 1987 in the name of Mitsuru Yamamoto, et al. According to this patent, the volume of an ejected ink droplet is varied by modulating electronic waveforms which activate ink droplet formation and ejection. These individual droplets are ejected from an ink jet nozzle and land separately on a receiver medium. This patent also discloses that ink volume can be modulated by a single electric pulse waveform, or by a plurality of electric pulse waveforms to control the operating dynamic printing range of the printer. However, the technique disclosed by this patent increases printing time and ink laydown due to increased levels of discrete optical densities.
U.S. Pat. No. 4,959,659 titled "Color Picture Timing Apparatus" issued Sep. 25, 1990 in the name of Takashi Saski, et al. discloses yet another method for increasing available levels of optical densities. According to this patent, a plurality of inks of different densities for each color are deposited on a receiver medium. Thus, an increase in printable ink densities is obtained by an increased number of available ink densities for each color. In addition, this method allows a plurality of ink droplets to be printed at each pixel location on the receiver medium by means of increased ink laydown. The disadvantages associated with this approach are the increased complexity and cost occasioned by the increased number of inks and ink cartridges in the printer. Another disadvantage of this method is that it results in increased ink laydown on the receiver medium.
The techniques disclosed by the art recited hereinabove print ink droplets at discrete optical densities. The techniques disclosed by the art recited hereinabove also cause quantization errors. Hence, these methods do not eliminate artifacts (i.e., extraneous visible discrete dots in the printed image). The techniques disclosed by the art recited hereinabove also increase ink laydown and printing time.
Another problem associated with prior art ink jet printers is image artifacts and visible noise caused by manufacturing variability among the plurality of ink jet nozzles comprising the print head. More specifically, variability in the print head fabrication process produces physical variability in the nozzles (e.g., variability in the size of nozzle orifices) comprising the print head. This variability in turn causes undesirable variability in ink droplet volume for droplets ejected from the nozzles. As a result, some nozzles will unintentionally print higher densities than other nozzles. Such variability causes visible noise and image artifacts, such as banding or streaks, along the direction of travel of the print head relative to the receiver medium. To reduce such visible image artifacts, some prior art ink jet printers resort to printing in multiple shingled passes over the same image area in a fashion such that each image location on the receiver medium is printed by different nozzles in each pass. In this case, print variability between nozzles is averaged out and the visual artifacts appear reduced. However, a disadvantage of this technique is that print time is substantially increased by a factor approximately equal to the number of passes.
A further problem associated with prior art ink jet printing devices relates to placement of the ink droplets on the receiver medium. More specifically, accuracy of ink droplet placement on a receiver is determined by, among other factors, the speed of ejected ink droplets and the duration of activation of the ink nozzles ejecting the ink droplets. In ink jet printers printing variable ink droplet volumes, speed of ejected ink droplets and the duration of the activation of the ink droplets can vary as a function of ink droplet volume. Variation in speed and activation of ink droplets often cause errors in placement of ink droplets having different volumes, thereby producing image defects such as ink coalescence or color bleeding on the receiver, unprinted white spots (sometimes referred as "pin holes") in a printed area, or unsharp lines and borders.
Therefore, there has been a long-felt need to provide an ink jet printing apparatus and method for variable gray scale printing while eliminating image artifacts caused by "quantization errors", visible noise and excessive ink laydown, and while also reducing printing time and improving accuracy of ink droplet placement on the receiver medium.